Gel electrophoresis is a process that has long been in use for clinical diagnosis and laboratory research. It is based upon the principle that electrically charged biological macromolecules will migrate through a solvent medium when subjected to an electrical field. Since macromolecules may vary in molecular weight and charge, it is possible to use an electrophoresis process to distinguish between different macromolecules based on their respective rates of movement through the solvent. Electrophoresis can also be used for other types of macromolecular analysis, such as detecting amino acid changes.
In a common form of gel electrophoresis, the gel solution is cast and solidifies into a thin planar slab gel. The samples being tested are placed within cavities or wells formed within the electrophoresis gel. The gel is then placed in a buffer solution within an electrophoresis gel chamber. A current is applied to the buffer solution causing the biological macromolecules to migrate through the gel.
Originally, the laboratories conducting the testing mixed the gel solution and cast their own gel slabs on-site. It soon became apparent, however, particularly as electrophoresis testing of DNA increased, that it is more convenient and more precise to use precast gel slabs made to uniform composition, size and configuration standards. The most common precast gel slab has a thin planar rectangular shape and includes a series of spaced wells which receive the biological samples being investigated. Conventional gel slabs are inherently flimsy and subject to tearing and deformation if not handled carefully. A particularly sensitive area in the gel is the thin walls separating the sample wells. While any deformation or tearing of the gel slab creates some risk of producing inaccurate results, a breach between wells allowing commingling of adjacent biological samples could generate erroneous results.
In many cases, the precast gel slabs are supplied in a package that cannot be placed directly into the gel chamber or which does not include a convenient mechanism for holding the gel submerged under the buffer solution within the gel chamber. Since the gel is nearly the same density as the buffer solution, small movements of the chamber can easily cause the gel to shift. Also, any slight movement of the overlaying buffer solution can cause the gel to shift. The motion of the buffer solution can be caused by thermal gradients produced in the buffer by the electric current, or by bubble generation in the buffer.
One method that has previously been used to anchor the gel involves a backing sheet adhesively attached to a flat surface of the slab. The sheet extends beyond the edges of the slab to form a narrow overhang of sheet along the sides of the slab. A plastic anchor having two long thin beams, connected at the ends and having a thin bridge in the middle, is placed over the slab such that the long beams rest on the sheet overhang. The device is sold under the brand name "Catamaran". The Catamaran type anchor, however, anchors the backing sheet rather than the gel directly. As such, if the gel slab becomes dislodged from the backing sheet when power is switched to the electrodes, the slab will float in the buffer liquid and cause a loss of sample from the wells or skewed electrophoretic patterns. In addition, the plastic backing sheet adhered to the gel precludes transfer of DNA to a solid support such as a nylon membrane, and can make DNA recovery more difficult.
Other than the Catamaran anchor described above, the object frequently used to anchor the bare slab is usually some handy laboratory device selected ad hoc, such as a glass rod or glass plate that is placed across a portion of the slab. While this is a practical solution, it is clearly not an optimal one, since such devices often cause distortions in the migration of macromolecules through the gel by interfering with the electrical field.
Another device recently developed to hold down a gel slab during an electrophoresis process is an anchor disclosed in co-pending application Ser. No. 09/178,218, entitled "Anchor for Electrophoresis Gel", filed Oct. 23, 1998. The anchor includes a plurality of supporting members (legs) which extend downward from a frame. The supporting members are positioned so as to rest on the top of the gel during the electrophoresis process.
Recently there has been an increase in the use of trays for supporting and transporting electrophoresis gels. These trays are typically placed within the chamber electrophoresis. While the trays provide a suitable mechanism for protecting the electrophoresis gel from damage, they increase the chances of developing artifacts in the gel. In particular, the use of a tray during an electrophoretic run can produce an effect known as "hourglassing", as well as result in the appearance of tilted bands in the gel. A schematic top view of an electrophoretic process is illustrated in FIG. 1. A gel 2 is shown positioned within a tray 4. Lines 6 graphically represent the electrical field flowing through the buffer and gel from one side to the other (e.g., left to right). The flow of the electric current over the side walls 4.sub.S of the tray 4 about midway along the length of the tray is wavy, as shown. As a consequence, the gel is subjected to non-parallel field lines in this area producing an hourglass shaped distortion in the sample lanes.
A similar phenomenon occurs in the front and back of the gel as the electric field passes over the front and back edges 4.sub.F, 4.sub.B of the tray 4. Referring to FIGS. 2, a schematic partial cross-sectional view of the tray is shown depicting the electrical fields generated over the front edge 4.sub.F during an electrophoresis process. This flow pattern results in an uneven current flow through the gel resulting in the samples traveling slower along the bottom of the gel than the top. As a consequence, the samples produce tilted bands (often perceived as fuzzy bands). Although the uneven current flow tends to diminish as the current flows through the gel, the cant introduced at the front of the tray continues through the whole gel.
Another problem with conventional trays is that they generally have the same density as the buffer solution and, thus, the same drifting problems described above associated with the gels can also occur with the use of trays.
A need, therefore, exists for an improved device for holding a gel in place during an electrophoresis process and which device minimizes inaccuracies caused by hourglassing and tilted bands.